A variety of electronic circuit substrates with a porous fluororesin sheet as an insulating layer are known, including flexible substrates. With these substrates, there have been substantial difficulties in forming a metal plating layer that adheres strongly to the porous fluororesin sheet. A variety of methods have been proposed to solve the problem. For example, for the purpose of raising the adhesive power between a metal plating layer and the through-hole interior walls in a substrate possessing through-holes, a method has been proposed wherein carbon is exposed through alkali metal etching of the through-hole interior wall surfaces, a plating catalyst such as a palladium salt is applied to the carbon, and then a conductive metal is chemically plated on the catalysed surface as shown, for example, in Japanese Laid-Open Patent Applications 59-72472 and 60-225750.
However, with these methods, the adhesive power between the through-hole interior wall surfaces and the plated metal is still inadequate, and defects arise, such as the separation of the plated metal on the interior of the through-holes and barrel cracking, wherein the metal plating layer breaks. Furthermore, by exposing carbon surfaces, electrical characteristics of the substrate at high frequencies are worsened.
Moreover, with electronic circuit substrates and other manufactured products containing a porous fluororesin sheet as an insulating layer, when a metal plating layer is formed on the sheet surface, the adhesive strength between the sheet surface and the metal plating layer is still inadequate.
The present invention overcomes these difficulties and provides a manufactured product wherein the adhesive power between a porous fluororesin sheet insulating layer and a metal plating layer is raised.